Conventional frequency counters operate by gating a cyclical signal for a predetermined period and counting the number of cycles during that period. This basic method is not reliable when the signal is seriously noisy or otherwise impaired and/or is of limited duration. In order to measure the frequency of such signals, it has become customary to use the signal to lock a phase-locked loop and to measure the loop parameters rather than the signal parameters in order to determine the periodicity of the latter. This method is capable of giving greatly improved results with a noise impaired signal, but still possesses some disadvantages, particularly with signals of limited duration.
One common type of input signal consists of individual bursts of oscillation with a rapid attack and a slow exponential decay. Such a signal is for example provided by proton precession magnetometers, the accuracy of such a magnetometer being commonly determined by the accuracy with which the frequency of the output can be ascertained. If a phase-locked loop is locked to such a signal, the locking process commences during the initial portion of the burst when its amplitude is highest, but before the locking process is completed, the signal amplitude will have dropped substantially and the influence of any noise present in the signal will be proportionately increased. Such noise may either prevent locking or render it impossible to be certain that locking was achieved. In any event, it is impossible to use the highest amplitude portion of the signal for measurement purposes, thus resulting in a significant diminution in both the length and the quality of the signal sample available. Further problems can occur when the quality or strength of the signal fluctuates or changes, so that locking is only achieved very briefly. In such cases, the accuracy of the result may be significantly impaired, but there may be nothing to indicate the quality of the signal upon which the measurement was made.
Difficulties may also arise where the signal contains components at closely adjacent frequencies such as to produce a beat effect. This will affect locking of the loop since the signal strength will fluctuate at the beat frequency and may thus fall below the level at which locking can be achieved after a short period quite unrelated to the duration of the signal.
Obviously, when dealing with an impaired signal of limited duration, it is important that the phase-locked loop is tuned as closely as possible to the frequency of the signal to which it is to be locked, and this is a further disadvantage of the arrangement in that the locking time will depend on the accuracy of the tuning.